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This article is a taxonomic revision of the Japanese bat fauna based on an extensive study of 4380 specimens collected from Hokkaido, Honshu, Shikoku, Kyushu, Rishiri I., Sado I., Oki I., Seven Is. of Izu, Tsushima Is., Goto Is., Iki I., Yakushima I., Kuchinoerabu I., Amami Is., Okinawa I., Daito Is., Kume-jima I., Ishigaki I. Iriomote I., Yonakuni I. and Ogasawara Islands. In this study, the following species concept was employed; the species is a population reproductively isolated from other similar populations, and the subsecies is a local population more or less clearly separated geographically and morphologically from the other populations of the species. Besides the external and internal features customarily used in chiropteran taxonomy at the species level, in most cases in this study such characters as the baculum, scapula, and sternum were also examined, to determine variability and the taxonomic status of the population as objectively as possible. Quantitative characters of local populations, such as mean values and standard deviations in length were compared statistically, always considering the nature of geographical trends or clines among of the population. On the other hand, in comparing of qualitative characters, special attention was paid to age variation. Where a group of local populations showed a distinct and smooth cline in an important quantitative character, then the populations within the group were recognized as members of a same species. But a local population clearly deviating from such a cline was recognized as specifically different because the population seems to be reproductively isolated from any others. By these methods, the Japanese chiropterans were divided into 44 species and subspecies, belonging to thirteen genera, five families, and two suborders. The chiropteran fauna of Japan is composed of three species of Pteropus in the family Pteropodidae of the Megachiroptera and 35 species of the Microchiroptera; five species of Rhinolophus in the family Rhinolophidae, one species of Hipposideros in the family Hipposideridae, twelve species of Myotis, five species of Pipistrellus, two species of Nyctalus, two species of Eptesicus, one species of Vespertilio, one species of Barbastella, one species of Plecotus, one species of Miniopterus and three species of Murina in the family Vespertilionidae, and one species of Tadarida in the family Molossidae. During this study following six new species were discovered from Japan, including two forms in collaboration: Rhinolophus imaizumii in the Rhinolophidae and Myotis yesoensis, pruinosus, Nyctalus furvus, Murina silvatica and M. tenebrosa in the Vespertilionidae. New localities were given to Pteropus dasymallus daitoensis, Rhinolophus ferrumequinum nippon, R. cornutus cornutus, R. c. orii, Myotis fujiensis, M gracilis, M. ikonnikovi, M. ozensis, M. daubentoni ussuriensis, Nyctalus aviator, Eptesicus nilssoni parvus, Vespertilio superans superans, Barbastella leucomelas darjelingensis, Plecotus auritus sacrimontis, Miniopterus fuscus, Miniopterus schreibersi fuliginosus, Murina silvatica, Murina hilgendorfi and Tadarida insignis. Rhinolophus imaizumii from Iriomote I. was recognized as a new species by myself and J. E. Hill of the British Museum (Natural History). I had noticed this form while studying a vast number of samples of this genus collected from Japan, and it seemed to be different from any known forms of the group including R. perditus from Ishigaki I. This was confirmed by Hill after careful comparison of samples of this form and type specimens of this group in London. An examination of about ten specimens of Myotis pruinosus proved that this is the smallest species of the subgenus Leuconoe, having the forearm as short as that of M. ikonnikovi, the smallest species of the subgenus Selysius. It is similar in general respects to M. davidi of China, but much smaller. Nyctalus furvus is a member of the noctula group, but eleven specimens of this species from Iwate Prefecture are much smaller than N. noctula of Eurasia and N. velutinus of China and have different cranial and dental characters. There was a distinct gap between furvus and the latter two species, so I recognized it as a distinct species. Murina silvatica has long been misidentified as M. aurata ussuriensis or M. ussuriensis. As a result of a comparison of about thirty specimens of silvatica from Japan, the type specimen of M. ussuriensis in the Zoological Museum of Moscow State University, and M. aurata aurata from the Himalayas, the distinctness of silvatica became evident, so it was given a new name. Murina tenebrosa from Tsushima Is., is known from only one specimen. But this species is different from all other Asiatic members of the genus by the nearly naked dorsal surface of the uropatagium and intermediate dimensions of several important characters. The nearest form to this species may be M. florium of Flores I. The recently described Myotis yesoensis of the subgenus Selysius is a medium-sized form similar to M. hosonoi of Honshu in general features. This species is known from several localities in the Hidaka Mountains, Hokkaido, and was found together with M gracilis in a forest on Mt. Petegari, the type locality. It differs from gracilis in its shorter ear and in several important cranial and dental characters. It is larger but has a relatively shorter tail than M. ikonnikovi of the same subgenus, known from Nakano-shima I., Toya Lake, but has a shorter tail. The coefficient of difference in the length of tail between these two forms is remarkable. All of the forms known from Japan are described in order of family, genus, species, and subspecies, from primitive to advanced. Keys are given to all families and genera and to species in polytypic genera. The description of each species or subspecies contains detailed features of the skull and teeth, and in most cases covers also the features of the body skeleton and baculum which have not been fully described. The taxonomic status of each form is also fully discussed. The Japanese mainland region consisting of Hokkaido, Honshu, Shikoku and Kyushu, the Ryukyu region, and the Ogasawara region differ remarkably from each other in their species composition of bats. The Japanese mainland region, belonging to the north eastern Chinese subregion of the Palaearctic region,. has many species common to Eurasia but also several peculiar to the region. Both the Ryukyu region belonging to the Indo-Chinese subregion of the Oriental region, and the Ogasawara region, belonging to the Polynesian subregion of the Australian region, have only a few species, but most of them are endemic. One of the characteristic features of the Japanese chiropteran fauna is that the number of species is more numerous in the region north of the STEJNEGER line, containing Hokkaido and a half of Honshfi, than in the region south of the line, contrary to the tendency seen in continental Asia.